<style>
    .map-overlay {
        font: 12px/20px 'Helvetica Neue', Arial, Helvetica, sans-serif;
        position: absolute;
        width: 200px;
        top: 0;
        left: 0;
        padding: 10px;
    }

    .map-overlay .map-overlay-inner {
        background-color: #fff;
        box-shadow: 0 1px 2px rgba(0, 0, 0, 0.1);
        border-radius: 3px;
        padding: 10px;
        margin-bottom: 10px;
    }

    .map-overlay-inner fieldset {
        border: none;
        padding: 0;
        margin: 0 0 10px;
    }

    .map-overlay-inner fieldset:last-child {
        margin: 0;
    }

    .map-overlay-inner select {
        width: 100%;
    }

    .map-overlay-inner label {
        display: block;
        font-weight: bold;
        margin: 0 0 5px;
    }

    .map-overlay-inner button {
        display: inline-block;
        width: 36px;
        height: 20px;
        border: none;
        cursor: pointer;
    }

    .map-overlay-inner button:focus {
        outline: none;
    }

    .map-overlay-inner button:hover {
        box-shadow: inset 0 0 0 3px rgba(0, 0, 0, 0.1);
    }
</style>

<div id="map"></div>
<div class="map-overlay top">
    <div class="map-overlay-inner">
        <fieldset>
            <label>Select layer</label>
            <select id="layer" name="layer">
                <option value="water">Water</option>
                <option value="building">Buildings</option>
            </select>
        </fieldset>
        <fieldset>
            <label>Choose a color</label>
            <div id="swatches"></div>
        </fieldset>
    </div>
</div>

<script>
    var map = new mapboxgl.Map({
        container: 'map',
        style: 'mapbox://styles/mapbox/light-v10',
        center: [12.338, 45.4385],
        zoom: 17.4
    });

    var swatches = document.getElementById('swatches');
    var layer = document.getElementById('layer');
    var colors = [
        '#ffffcc',
        '#a1dab4',
        '#41b6c4',
        '#2c7fb8',
        '#253494',
        '#fed976',
        '#feb24c',
        '#fd8d3c',
        '#f03b20',
        '#bd0026'
    ];

    colors.forEach(function(color) {
        var swatch = document.createElement('button');
        swatch.style.backgroundColor = color;
        swatch.addEventListener('click', function() {
            map.setPaintProperty(layer.value, 'fill-color', color);
        });
        swatches.appendChild(swatch);
    });
</script>
